This invention relates to the control of gas flow from a pump through a resistive load at a constant selected flow rate without unacceptable effect of change in load resistance. A major application is in the sampling of environmental air for the purpose of measuring levels of airborne contaminants for protection against pollutant-related diseases. For a number of years personal and area sampler pumps have been used to draw air samples of known volumes through collection devices, such as filters, to collect particulates in the sampled air volume, and sorbent tubes to trap vapors and Gases for future analysis, as well as direct reading colorimetric indicator tubes. Pumps have also been used for direct collection of air samples for analysis. Although fixed volume grab samples are sometimes taken, these are usually for reasons of immediate safety, and for long-term health protection, the air sampled should be taken at a constant rate over an extended period of time to provide a time-weighted average measure of the contaminant concentration. Personal sampler pumps are designed to be worn by the individual being monitored for a number of hours so as to obtain a measure of the average concentration of contaminant breathed by an ambulatory worker or other individual at various locations.
The health hazard caused by airborne asbestos fibers is widely recognized, and various governmental regulations on the federal, state and local levels have been promulgated for the removal of asbestos from existing structures and vehicles. An application of the subject invention is for personal monitoring at sites of asbestos removal. The application is not limited to asbestos monitoring, however, as there are continuing hazards from other airborne dusts such as silica, cotton dust, and, more recently, airborne lead, which provide requirements for an improved air sampler.
There are certain limitaions of sampler pumps currently available. In most portable pumps the flow rate is set at the beginning of the sampling period by connecting the pump to an external meter at the beginning of the sampling period, and an inferential control is used to maintain constant flow during sampling. Also, where a flow indicator is supplied with the pump, it is usually of poor accuracy, such as a small rotameter, and it is located on the outlet of the pump where an erroneous indication can occur due to leakage in the pump and pneumatic line.
Baker and Clark in U.S. Pat. No. 4,063,824 show a control in which the pressure drop across a constant orifice (or valve) is maintained at a constant value by means of a pressure switch and integrator, which vary the pump speed. To change the flow rate, however, an external flowmeter must be connected, and the valve setting changed, a procedure which is difficult to accomplish satisfactorily in the field.
Lalin in U.S. Pat. No. 4,432,248, and Hollenbeck in U.S. Pat. No. 4,237,451 describe control systems in which the flow rate is manually set prior to sampling, and the flow rate is controlled by adjusting pump speed in relation to increase in motor current caused by loading of a (particulate) collection filter.
In U.S. Pat. No. 5,000,032 I have disclosed a controlled sampler in which the direct measurement of the true volumetric flow rate is used to set and control the flow rate of sampled air. It is not necessary to set the flow rate with an external calibrator or flowmeter prior to sampling. The sampler includes an accurate linear flowmeter so that flow rate can be precisely changed in the field and during sampling. This device has been used for area sampling and provides excellent performance. A drawback with this controlled sampler for application as a personal sampler is that the size and weight of the flowmeter are excessive. Also, a laminar flowmeter is used with a differential pressure transducer, whose range is limited for accurate readings to approximately 10:1; and the pressure drop required for accurate measurements with current semiconductor transducers will require larger batteries and additional weight for a personal sampler.
Betsill et al in U.S. Pat. No. 5,163,818 disclose a constant air flow rate pump for sampling air in which air flow rate is computed from measurements of voltage, current and motor speed. Computation of flow rate from pump characteristics is appealing, since it eliminates the size and weight attributed to direct flowmeters, and this means has been used precisely in my U.S. Pat. No. 4,957,107 for gas delivery means and used in a prototype wearable ventilator. It is doubtful, however, that more than short-term accuracy can be achieved from a computed value based on current drain because of the various energy loss mechanisms in addition to flow rate, such as friction, that can change the current.
There is a need for a constant flow rate pumping system that permits the accurate setting of flow rate at any time without need for prior setting with an external flowmeter or calibrator that has a relatively wide operating range, and that achieves this operation with a minimum number of components and minimum size and weight suitable for a personal sampler pump.